on natural pollen surfaces Hierarchical synthesis of corrugated photocatalytic TiO microspherearchitectures Applied Surface Science

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Applied Surface Science

j o u r n a l h o m e p a g e :w w w . e l s e v i e r . c o m / l o c a t e / a p s u s c

Full Length Article

Hierarchical synthesis of corrugated photocatalytic TiO 2 microsphere architectures on natural pollen surfaces

Deniz Altunoz Erdogan, Emrah Ozensoy

^∗,1

DepartmentofChemistry,BilkentUniversity,06800,Ankara,Turkey

a r t i c l e i n f o

Articlehistory:

Received12October2016

Receivedinrevisedform7January2017 Accepted12January2017

Availableonline17January2017

Keywords:

TiO2

Photocatalyst Ambrosiatriﬁda NO(g)oxidation RhodamineB

a b s t r a c t

Biomaterialsarechallenging,yetvastlypromisingtemplatesforengineeringunusualinorganicmaterials withunprecedentedsurfaceandstructuralproperties.Inthecurrentwork,anovelbiotemplate-based photocatalyticmaterialwassynthesizedintheformofcorrugatedTiO2microspheresbyutilizingasol-gel methodologywhereAmbrosiatriﬁda(Ab,Giantragweed)pollenwasexploitedastheinitialbiologicalsup- portsurface.HierarchicallysynthesizedTiO2microsphereswerestructurallycharacterizedindetailvia SEM-EDX,Ramanspectroscopy,XRDandBETtechniquesinordertoshedlightonthesurfacechemistry, crystalstructure,chemicalcompositionandmorphologyofthesenovelmaterialarchitectures.Photo- catalyticfunctionalityofthesynthesizedmaterialswasdemonstratedbothingasphaseaswellasin liquidphase.Alongtheselines,airandwaterpuriﬁcationcapabilitiesofthesynthesizedTiO2micro- sphereswereestablishedbyperformingphotocatalyticoxidativeNOx(g)storageandRhodamineB(aq) degradationexperiments;respectively.Thesyntheticapproachpresentedhereinoffersnewopportuni- tiestodesignandcreatesophisticatedfunctionalmaterialsthatcanbeusedinmicroreactorsystems, adsorbents,drugdeliverysystems,catalyticprocesses,andsensortechnologies.

1. Introduction

Hazardouschemicalsarisingfromcombustionoffossilfuels, suchassulphurdioxide,nitrogenoxides,mercury,aswellasindus- trialwastematerialincludingorganicdyes,andsolventsareamong theprominentcontaminantscontributingtothewater,airandsoil pollution;causingawidevarietyofseverehealthandenvironmen- talproblems[1–5].Heterogeneouscatalysisplays animportant roleincopingwiththeenvironmentalpollutionattheglobalscale.

Oneofthemostabundantrenewableenergysourcesthatcanbe exploitedinheterogeneouscatalysisapplicationsinanenviron- mentallyfriendlyandeconomicalmanneristhesolarenergy.Thus, thereexistsanimmensedemandtodevelopnovelphotocatalytic materialsusinginnovativesyntheticmethodologies[6–11].

Alargevarietyofmaterialssuchasmetaloxides,metalhydrox- ides,metalcarbides,metal nitrides,carbonallotropes and their derivativeshavebeeninvestigatedintheliteratureasphotocat- alysts[12–18].Amongthem,titaniumdioxide(TiO2)hasreceived considerableattentionsincethesuccessfulgenerationofH₂from waterviaelectrochemicalphotolysisof waterby Fujishimaand

∗ Correspondingauthor.

E-mailaddress:ozensoy@fen.bilkent.edu.tr(E.Ozensoy).

1 Web:http://www.fen.bilkent.edu.tr/∼ozensoy

Honda[19].TiO₂hasbeenthemostfrequentlyutilizedphotocat- alyticmaterialduetoitsfunctionalversatilityinawiderangeof processessuchasenergystorage/conversion,photocatalyticpollu- tionabatement,andbiotechnology[20–22].

It is well known that physical and chemical properties of materialssuchasshape, texture,particlesize,porosity,specific surfacearea,crystallinity,electronicbandgap,surfacedefectsand surfacefunctionalgroupsdirectlyinfluencethephotocatalyticper- formance. Particularly, shape and surface structural properties ofphotocatalytic materialscanbecloselylinkedtothereactiv- ity and selectivity of these systems [23–25]. One of the most efficientandsimpleapproachestopreparesophisticatedsurface structuresonmaterialsistemplating.Natural/biologicalstarting materialscanbeusedastemporalsupportsystems/sacrificialtem- platesinordertocreatewell-definedshapes,sizesandtextures onsurfaces.For instance,mesoporous hollowSnO₂ microfibers were prepared using natural kapok (Ceiba pentandra) fiber as a template and were found to be photocatalytically active in methylene blue dye degradation under UV irradiation [26]. In another study, freshnatural rose (Rosa hybrida L.)petals were used as a template to synthesizeTiO2 flakes exhibiting higher photocatalyticactivitythanthecommercialDegussaP25photocat- alyst[27].Also,cerium-dopedTiO₂mesoporousnanofiberswere preparedby a single-potsynthesis methodusing collagenfiber

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160 D.A.Erdogan,E.Ozensoy/AppliedSurfaceScience403(2017)159–167

biotemplates[28].A variety of synthetic techniqueshave been developedtodepositthedesiredphotocatalyticmaterialonthesur- faceofbio-templatesincludingsputtering,sol-gel,electrochemical deposition,andchemicalvapourdepositionapproaches.Usingsuch syntheticmethods,micro/nanostructuressuchaswires,tubes,rods orspherescanbefabricatedpreservingtheoriginalshapeandsize oftheinitialnaturaltemplate.Pollengrainsattractattentionasver- satilebiotemplatesduetotheiruniqueandsophisticatedsurface structuresatthemicro/nanoscale[7,29–33].

Thus,inthepresentstudy,asimplebiotemplateassistedsol- gelrouteispresentedinordertosynthesizeTiO2 photocatalytic microsphereswithuniquesurfacemorphologies,whereAmbrosia triﬁda (Ab, Giant ragweed) pollen is used as the starting bio- substrate. Ab is selected as a biotemplate due to its unusual micron-sizedsurfacemorphologyexhibitingconicalnano-spikes.

Upondetailed structural characterizationof this novelmaterial platform,photocatalyticfunctionalityofthesehierarchicalsystems underultraviolet-A(UVA)irradiationisalsodemonstratedattwo differentinterfacesnamely,RhodamineB(RhB)photodegradation attheliquid/solidinterfaceaswellasthephotocatalyticoxidative storageofNOx(g)atthegas/solidinterface;illustratingthecatalytic versatilityofthisnewfamilyofmaterials.

2. Experimental 2.1. Materials

Ambrosiatriﬁda(Ab,Giantragweed)pollenswereobtainedfrom Bonapola.s.Company(CzechRepublic).Titanium(IV)isopropoxide (TIP, 97%), ethanol (≥99.8%), and Rhodamine B (RhB, dye con- tent ∼95%)were purchased fromSigma-Aldrich (Germany).All chemicalswere of analytical grade and used as receivedwith- out any further treatment. Milli-Q ultra-pure deionized water (18.2Mcm)wasalsousedasasolvent.

2.2. SynthesisofbiotemplatedTiO₂microspheres

BiotemplatedTiO2microsphereswerepreparedusingamethod analogousto theone described in one of ourprevious reports [7].Briefly,Ab pollenswerewashedwithanhydrousethanolto removesurfaceimpuritiesandsubsequentlydriedunderambient conditionsfor48h. Then,titanium(IV)isopropoxide(TIP,4mL) precursorwasmixedwithethanol(2mL)foraperiodof10min atroomtemperature.100mg cleanAb pollen(i.e.,biotemplate) wasaddedtothepreparedprecursorsolutionandtheslurrywas stirredvigorouslyfor30min.Afterdepositingprecursorsolution ontheoutersurface(i.e.exine)ofthebiotemplate,themixture wasfilteredtoremovetheexcessdecantate.Coatedsamplewas agedfor60minunderambientconditionsinordertoallowforthe hydrolysisandpolycondensationreactionstoproceed,formingan amorphousTiO2shellonthebiotemplatesurface.Then,calcination stepswereexecutedinamufflefurnaceatvarioustemperatures varyingwithin400^◦C–900^◦C(for2.5hpercalcinationstep)inair, wherethesacrificialbiotemplatewaseliminatedandthecrystal- lizationandorderingoftheTiO2overlayerwereachieved.Products obtainedattheendofthesynthesisprotocolarenamedasAbTi-X, whereXindicatesthecalcinationtemperature.

2.3. Characterization

Surfacestructureandmorphologyofthesampleswereinvesti- gatedviaaCarl-ZeissEvo40scanningelectronmicroscope(SEM) withanacceleratingvoltagevaryingwithin5–10kV.Forelemen- talanalysis,energydispersiveX-ray(EDX)analysisofthepowder samplesdispersedonanelectricallyconductivecarbonﬁlmwas performedusinganacceleratingvoltageof10kV.

Crystallographicchangesonthesamplesaftercalcinationwere determined via XRD measurements performed using a Rigaku (Japan)X-raydiffractometerequippedwithaMiniﬂexgoniometer andamonochromatedhigh-intensityCuK␣radiation(␭=1.5405Å, 30kV,15mA)source.XRDdatawerecollectedbyscanningthe2␪ rangewithin10–60^◦ usingastepsizeof0.02^◦s⁻¹.Identiﬁcation oftheunknownphasesin thepowder XRDdataweremadeby utilizingPowderDiffractionFile(PDF)databasemaintainedbythe InternationalCentreforDiffractionData(ICDD).

RamanexperimentswerecarriedoutusingaLabRAMHR800 spectrometer(HoribaJobinYvon,Japan)equippedwithaNd:YAG laser(␭=532.1nm,20mW)andanintegratedconfocalOlympus BX41microscope.Thesystemwascalibratedusingthereference Si Ramanshiftat 520.7cm⁻¹ byadjusting the zero-orderposi- tionofthegrating.Powdersamplewasevenlyspreadonasingle crystalSiwaferandRamanspectrawererecordedintherangeof 100–1500cm⁻¹withaspectralresolutionof4cm⁻¹atroomtemperature.

TheBrunauer-Emmett-Teller(BET)SSAmeasurementsofthe synthesized catalystsweredetermined by nitrogenadsorption- desorptionisothermsusingaMicromeriticsTristar 3000surface areaandporesizeanalyser.PriortoSSAanalysis,allsampleswere outgassedinvacuumfor2hat150^◦C.

2.4. Liquidphasephotocatalyticactivitytestsforthedegradation ofRhB(aq)

Photocatalyticfunctionality of thebiotemplatedTiO₂ microspheres in liquid phase was demonstrated via RhB (aq) dye degradationunderUVA irradiationat roomtemperature.RhB is afrequentlyusedmodelpollutantfortesting thephotocatalytic activityofnovelmaterialsinwater.RhBdegradationexperiments wereperformedinaphotocatalyticreactorequippedwithan8W SylvaniaUVA-lamp(F8W,T5,Black-light,368nm).Acoolingfan wasalsoinstalledinsidethereactorfortemperatureregulation.Ini- tially,a48mLaqueoussolutionofRhB(10mgL⁻¹)wasprepared indarkand25mgofbiotemplatedTiO₂microsphereswereultra- sonicallydispersedinthissolutiontoformasuspension.Then,the samplecontainerwasplacedataspeciﬁedpositioninsidethepho- tocatalyticreactor,wherethedistancebetweenthelightsourceand thesuspensionwasﬁxedat13cm.PriortoUVAlightirradiation, thesuspensionwasmagneticallystirredinsidethereactorunder darkconditions for 30minin order toestablish anadsorption- desorptionequilibriumbetweenthephotocatalystandRhB(aq).

BeforetheUVAlightexposure,a3mLaliquotwasextractedfrom thesuspensionunderdarkconditionsandtheconcentrationofthis startingsolutionwasdesignatedasC₀. Then,identicalamounts ofsampleswereobtainedduringtheUVAlight irradiationafter certaintimeintervalswhoseconcentrationsweredenotedasCt.

Afterremovingthephotocatalystfromtheextractedsamplesvia centrifugation,RhBconcentrationoftheextractedsolutionswere determinedusingaUV–visspectrophotometer(Carry300,Agilent) withthehelpofacalibrationcurveutilizingtheRhBcharacteristic maximalabsorptionbandatca.553nm.Thetypicalphotonpower density(irradiance)duringtheexperimentswas7.4Wm⁻²which wasmeasuredbyaphotoradiometer(DeltaOhm,HD2302.0,Italy) equippedwithaUVAprobe(DeltaOhm,LP471UVA).Thephotocat- alyticdyedegradationefﬁciency(D_eff)ofthephotocatalystswas calculatedaccordingtofollowingequation;

D_eff(%)= (C0−Ct)

C₀ ×100 (1)

where,C0istheinitialRhBconcentrationandCtistheRhBconcen- trationatagiventimet.

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Fig1. (a)EDXspectrumofthebare(uncoated)Abpollenobtainedfromtheblue-colouredcircularregionin(b).(b)SEMimageoftheuncoatedAbpollen.(c)Schematic describingthevariousbio-structuralsectionsoftheAbpollen.(d)EDXspectraobtainedaftercalcinationoftheuncoatedAbpollensat800^◦C.Redandblackspectrawere obtainedfromthecircularregionswiththecorrespondingcolourspresentedin(e).(e)SEMimageoftheuncoatedAbpollensaftercalcinationat800^◦C.(Forinterpretation ofthereferencestocolourinthisﬁgurelegend,thereaderisreferredtothewebversionofthisarticle.)

2.5. Gasphasephotocatalyticactivitytestsfortheremovalof gaseousnitricoxide

Inordertodemonstratethefunctionalversatilityofthesyn- thesizedbiotemplatedTiO₂microspheres,inadditiontotheliquid phase photocatalytic tests, obtained materials were also used inphotocatalyticoxidative storageofNO atthesolid/gasinter- face. Photocatalytic removal of NO(g) over biotemplated TiO₂ microsphereswasperformedatroomtemperatureinacustom- madecontinuousphotocatalyticflowreactorwhichwasdesigned consideringtheISO22197-1:2007standard[8–10,34].Thispho- tocatalyticreactionsystemwascomposedofa gassupply unit, aflat-bedphotoreactorchamberhousingthesample,aUVAillu- minationsourceand a chemiluminescentNOxanalyser (Horiba APNA-370) for continuous inline monitoring of the NO, NO₂ andtotalNOxconcentrations[8–10].Inthegassupplyunit,NO (100ppm NO in N₂ balance, Linde GmbH) was mixed withO₂ (99.998%,LindeGmbH) andN₂ (99.998%, LindeGmbH)atroom temperature.Thetotal gasflow rateinthereactorwaskeptat ca.1.0SLM(standardlitersperminute)viamassflowcontrollers (MFCs, MKS,1479A)byadjusting theflowrateof eachgas(i.e.

N2=0.75 SLM,O2=0.25 SLM, and NO=0.01 SLM).The gasmix- turewasalsopassedthroughawaterbubblerbeforethereactor forhumidiﬁcationandtherelativehumidity(RH)ofthegasmix- turewasmeasuredviaaHannaHI9565humidityanalyzeratthe samplepositionin thephotocatalytic reactoratroomtempera- ture.RHwasdetectedtobetypicallyca.70%atroomtemperature duringthemeasurements.Synthesizedphotocatalystpowdersam- ples(250mg)weregentlypressedonapoly-methylmethacrylate (PMMA)sample holder (2×20×20mm³) toproduce a smooth

surface. Inorder toactivatethephotocatalysts and remove the initialsurfacecontaminants,beforethegasphasephotocatalytic activitymeasurements,sampleswereexposedtoUVAirradiation underambientconditionsfor18h.Then,thesamplewasplaced intothephotocatalyticreactor,whereaUVAlamp(SylvaniaUV- lamp,black-light,F8W,T5,368nm)wasplacedabovethereactor.

Next,thegasmixturewasfedtothephotocatalyticreactor,where thegasfeedsweptthesurfaceofthepowderphotocatalystsample.

Afterestablishingtheadsorption-desorptionequilibriuminsidethe photocatalyticreactor,UVAilluminationsourcewasactivatedto initiatethephotocatalyticreaction.Controlexperimentscarried out intheabsence ofa photocatalyst(i.e.in theempty reactor undertheUVAillumination)revealednocatalyticconversion.Pho- tocatalyticconversion efﬁciencyfor NOand photocatalyticNO₂ productionefﬁciency(␨%)overTiO₂microsphereswerecalculated asfollows:

%=n_NO_xorn_NO₂

n_photon ×100 (2)

where, nNOx is thedecreaseinthetotal numberof molesof all gaseous NOx species and n_NO₂ is the number of moles of NO₂ generatedin 60min(i.e.over thecourseofafullphotocatalytic NOxremovalexperiment).Inthisequation,n_photoncorrespondsto thetotalnumberofmolesofincidentUVAphotonsimpingingon thecatalystsurfaceduringthe60mintime interval.n_photon was calculatedby usingthephoton powerdensityoftheUVAlamp (I=7.4Wm⁻²), representativeemission wavelengthof the UVA lamp(␭=368nm),surfaceareaofthesampleholderthatisexposed to theUVA irradiation (S=2cm×2cm=4cm²), duration of the

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Fig.2.SchematicillustrationofoneofthepossiblesyntheticroutesleadingtotheformationofbiotemplatedTiO2microspheres:(i)coatedAbpollen,(ii)biotemplatedTiO2

microspheresaftertheremovaloftheAbpollenbycalcination.

photocatalytictest(t=3600s),Avogadro’snumber(NA),Planck’s constant(h),andthespeedoflight(c)asshowninEq.(3)below:

n_photon= ISt

N_AhC (3)

3. Resultsanddiscussion 3.1. Structureandmorphology

Surface elemental composition and the morphology of the uncoatedAbpollengrainswereinvestigatedusingSEMandEDX techniques(Fig.1aandb).Macroscopicstructuralcomponentsof theAbpollenswerealsoschematicallydescribedinFig.1c.Ascan beseeninFig.1c,Abpollensarecomposedoftwonestedlayers coveringthelivingmatterandprotectingitagainsttheexternal physicaland chemical adverseeffects [35,36].The robustouter surfacecalledexine(Fig.1c)iscomposedofahighlycrosslinked organicsubstancethatcanincludefattyacids,phenylpropanoids, andphenolicsporopollenins.Theinnerlayerofthepollen(Fig.1c)is calledtheintineandisprimarilycomposedofcellulosicmaterials andpolysaccharides[35,36].Inordertostudythemorphological andstructuralalterationsoccurringuponcalcination,uncoatedAb pollensampleswereinvestigatedcomparativelybySEMandEDX analysisbeforeandaftercalcination(Fig.1a–e).Fig.1bshowsthat, uncoatedAbpollenshaveasphericalshapewithanaveragepollen sizeof23.5±1.5␮mdecoratedwithconicalnano-spikes/thorns.

Afterthecalcinationoftheuncoatedpollensat 800^◦C, obvious structuraland morphological changes wereobserved signifying visible geometric deformation (Fig. 1e). According to the EDX spectragiveninFig.1a,whileuncoatedmicrosphereshaveacar- bonaceousoutermostlayerexhibitingmainlyCandOsignalsbefore calcination,uponcalcinationat800^◦C,pollensseemtolosetheir structuralintegrityanddeformfromtheiroriginalshapes,reveal- ingavarietyofEDXsignalscorrespondingtoelementssuchC,O, Mg,P, S, K,and Ca(Figs.1dand e). It islikely that duringthe calcinationprocess,outerexinelayerof theuncoatedpollensis partiallydestroyedandthebiologicalmaterialinsidetheintinecap- sule,whichmayinvolvevariousmineralsforvitality,diffusetothe

Fig.3.SEMimageandthecorrespondingEDXspectrumoftheAbbiotemplateafter titanium(IV)isopropoxide(TIP)depositionat25^◦C(AbTi-25).

surfaceatelevatedtemperatures,leadingtothedetectionofthe EDXelementalsignalsforC,O,Mg,P,S,K,andCa(Figs.1dande).

Inthecurrentwork,Abpollenswereselectedasabiotemplate todirecttheformationofbiomorphicTiO2microspheresusingthe

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sol-gelprocess.Fig.2providesoneofthepossiblereactionpath- waysforthesol-gelsyntheticrouteusedherein.Asillustratedin Fig.2,after thealcoholysisreaction,metalalkoxidespecies are expected tobind tothe naturally functionalizedsurface of the pollentemplate throughcondensationandhydrolysis reactions.

Extent of themetalalkoxide depositionand thecorresponding thicknessoftheultimateTiO₂ overlayerwerecontrolledbythe composition/concentrationoftheprecursorsolutionaswellasthe durationofthealcoholysisreaction.AftertheformationoftheTiO₂ overlayer,calcinationprocessleadstotheformationofabiomor- phicTiO2surfacepreservingmostoftheoriginalshape,size,and morphologyoftheAbbiotemplate.

Fig. 3 shows the SEM image and the corresponding EDX spectrum of the TiOx deposited Ab pollens after hydrolysis andpolycondensationreactionsatroomtemperature(i.e.before calcination). SEM image reveals the formation of a homoge- neous/continuous TiOx overlayer preserving the characteristic microstructureofthenascentpollensurface.Thisisalsoevident bytheEDXspectruminFig.3,indicatingastrongTisignalover- whelmingthatoftheotherpre-existingelementsonthesurface suchasCa,S,K,andP.

Calcination was employed in order to convert amorphous TiOx coating onthe Ab pollensinto crystallineTiO₂ overlayers (Fig.4).Low-magniﬁcationSEMimage(Fig.4a)shows thatTiO2

microspheresare relativelywell-dispersed rather than severely aggregated.Whilethecalcinationprocessinducesthecrystalliza- tionoftheTiOxoverlayertoTiO₂,italsoleadstomorphological modiﬁcationsatthenanometerscaleresultingintheformationofa spongy/porousandacorrugatednetworkonthesurface(Fig.4b–d andf).ComparisonofthebareAb(Fig.1b)orcoatedAbpollens beforecalcination(AbTi-25,Fig.3),withtheonesobtainedaftercal- cination(e.g.600^◦Cand800^◦C)suggestsdeformationofthesharp conicalspikes(Fig.4b–dandf),inadditiontotheshrinkingofthe pollenstoasmalleraveragediameterofca.13␮m.

Aftercalcination(Fig.4e),P,K,andCasignalsoriginatingfrom thebiotemplatebecomesdiscernibleontheAbTi-600andAbTi-800 surfaces.ItcanbeseeninFigs.4band4ethatanaperturewith anapproximatediameterof2.5␮mexistsintheAbpollenstruc- ture,fromwhichthepollentubeextendsatgerminationtofertilize theovum.Thus,itisfeasiblethatduringthecalcinationprocess,the interiorpartofthepollen(i.e.intineandotherbiologicallivingmat- terdepictedinFig.1c)diffusesoutboundthroughthisapertureand spilledoverontheTiO₂surfaceatelevatedtemperatures.

Fig.5aandbillustratethephasechangesoccurringontheAbTi materialsasa function ofcalcinationtemperature viaXRDand Ramanspectroscopy,respectively.Itisapparentthatthediffraction signalsinFig.5a intensifyand sharpenwithincreasingcalcina- tiontemperaturessuggestingorderingandcrystallizationofthe TiO₂ overlayer on the AbTi surface. XRD patterns of the AbTi microspheresrevealpredominantlyanatasephase atcalcination temperatures≤600^◦C;while twodifferentorderedTiO2 phases namely,anatase(ICDDNo.00-021-1272)andrutile(ICDDNo.00- 021-1276)arevisibleforthecalcinationtemperaturesabove600^◦C (Fig.5a).Also,Fig.6depictsrelativemassfractionofanataseand rutilephasesforvarioussamplescalculatedusingtheXRDdatavia SpurrandMyersapproach[37].Itisclearthattheanatasetorutile phasetransitionontheAbsurfacestartstooccurpredominantlyat T≥800^◦C.RutilemassfractionincreasesdrasticallyforT≥800^◦C, whilefortheAbTi-900sample,anataseandrutilephasesreveal almostequalmassfractions.

Averagecrystallitesizesoftheanataseandrutilephaseswere alsocalculatedusingtheanatase(101)andrutile(110)diffraction signalsviaScherrerequation[38,39](Fig.6).Theseresultssuggest thatanataseandrutiledomains havesimilaraveragecrystallite sizes(ca.20–30nm)forAbTi-700andAbTi-800sampleswhilethey drasticallydivergefromeachotherfortheAbTi-900sample,where

rutilecrystallitesizesurpassesthatoftheanatase(ca.47nmfor anataseandca. 134nm forrutile).Fig.6indicatesthatincreas- ingcalcinationtemperaturesresultsinamonotonicincreaseinthe crystallitesizesofanataseandrutiledomainsduetosintering.

Ramanspectroscopic measurementswerealso performedin ordertoconﬁrmthestructuralpropertiesofthebiotemplatedTiO₂ microspheres.Fig.5bdisplaystheRamanspectraofthesamples preparedby calcinationof thecoatedAbTi samplesat different temperatures.CharacteristicanataseRamanscatteringfeaturesat 147cm⁻¹(Eg),397cm⁻¹(B1g),515cm⁻¹(A1g),and641cm⁻¹(Eg) areobservedforallsamplesexcepttheAbTi-400sample.Forthe AbTi-900sample, additionalRamanpeaks at445cm⁻¹ (Eg)and 612cm⁻¹ (A1g)arevisiblewhichcanbeattributedtotherutile phase.In goodaccordancewiththecurrent XRDmeasurements (Fig.5a),RamandatainFig.5balsoindicatethattherutilecontent ofthesamplesincreaseswithincreasingcalcinationtemperatures.

Inadditiontothesesignals,someoftheRamanspectrainFig.5balso includesanadditionalfeatureat484cm⁻¹(labelled“withthesym- bol“”inFig.5b)whichcantentativelybeattributedtocomplex temporalspeciesgeneratedduringthecalcinationofthebiopoly- mermatrixoftheunderlyingAmbrosiatemplate[7].

3.2. PhotocatalyticactivityofthebiotemplatedTiO₂microspheres

Fig.7apresentsthephotocatalyticRhB(aq)degradationstudies performedunderUVAirradiationatroomtemperaturebyusing biotemplatedTiO₂microspherescalcinedatvarioustemperatures.

Fig.7bshowsatypicalseriesoftime-dependentUV–visabsorp- tionspectraoftheRhB(q)containingtheAbTi-800sampleobtained duringtheUVAirradiation.ItisapparentthatthecharacteristicRhB absorptionbandat553nmgraduallydecreaseswhilethephotocat- alyticdyedegradationreactionproceeds.After120minUVAlight exposure,colororiginatingfromRhBdyeisvirtuallydisappearsevi- dentbythevanishingabsorptionsignalat553nm.Notethatthe photocatalyticRhB(aq)degradationperformanceoftheAbTi-400 sampleisnotreportedinFig.7.Thisisduetothefactthatsuchlow calcinationtemperaturesdonotallowthecompleteremovalofthe biotemplatewhichinturn,leadstotheformationofgrainswithlow materialdensitythatcanﬂoatonthetopoftheRhB(aq)solution preventingtheirhomogenousmixinganduniformirradiation.

Asastandardcontrolexperiment,measureddecreaseinRhB concentrationoftheRhB(aq)solutionunderUVAirradiationinthe absenceofacatalystwasalsomonitoredinordertoinvestigatethe non-catalyticselfphotodegradationoftheRhBdye(Fig.7a).Ascan beseeninFig.7a,within500–800^◦C,increasingcalcinationtem- peratureleadstoamonotonicenhancementinthephotocatalytic RhB(aq)degradation.However,calcinationathighertemperatures suchas900^◦CresultsinanattenuationofthephotocatalyticRhB (aq)decompositionperformance.Basedonthestructuralcharac- terizationdataprovidedinFig.6andtheliquidphasephotocatalytic activitydatagiveninFig.7,itcanberealisedthattheoptimum photocatalyst samplefor RhB (aq) degradation (i.e.AbTi-800)is comprisedofbothanataseandrutiledomainswithaspeciﬁcsur- faceareaofca.7–8m²/g.Itisalsoapparentthatthemonotonic increaseinthephotocatalyticRhB(aq)decompositionperformance within500–800^◦Cisconcomitanttotheincreaseinthecrystallinity aswellastheaveragesizeoftheanatasedomains,wherethelatter convergestoca.30nmfortheAbTi-800sample.Fig.6alsosuggests thattheAbTi-800sampleiscomprisedof94.9%anataseand5.1%

rutilebymass.Ontheotherhand,atelevatedcalcinationtempera- turessuchas900^◦C,relativerutilemassfractionincreasestovalues abovetheoptimalvalue,leadingtoattenuationinthephotocat- alyticperformance(Figs.6and7).Itisapparentthattheoptimum bio-templatedphotocatalyststudiedinthecurrentworkforthe RhB (aq) degradation processes possesses co-existing anatase and rutile domains functioning in a synergistic manner with

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Fig.4.SEMimagesofthebiotemplatedTiO2microspherescalcinedfor2.5hinairat(a)800^◦C(lowmagniﬁcationimage,AbTi-800)(b)600^◦C(AbTi-600),and(c,d,and f)800^◦C(AbTi-800).Image(d)alsoemphasizestheSEMimageshowingthedetailedmorphologyoftheAbTi-800pollensurfaceexhibitingaporousandacorrugatedTiO2

overlayerstructure.(e)EDXspectraobtainedfromthecircledregionslabelledas1,2,3in(f).

Fig.5.(a)XRDpatternsand(b)theRamanspectraofthebiotemplatedTiO2microspherescalcinedat400,500,600,700,800,and900^◦Cfor2.5hinairaftercoating.“A”and

“R”letterscorrespondtoanataseandrutilephases;respectively(seetextfordetails).

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Fig.6.VariationoftheanataseandrutileaveragecrystallitesizesandmassfractionsonthecoatedAbTisamplesasafunctionofcalcinationtemperature.

Fig.7. (a)PhotocatalyticRhB(aq)degradationperformanceofbiotemplatedTiO2microspheresunderUVAilluminationatroomtemperature.Measurementlabelledasthe

“RhBSolution”wasperformedintheabsenceofaphotocatalystunderUVAirradiation.(b)Time-dependentUV–visabsorptionspectraoftheAbTi-800sampleduringthe photocatalyticRhB(aq)degradationprocess.

particularcrystallitesizesandauniquemassfraction.Thisobser- vationisinperfectagreementwithformerphotocatalyticstudies onotherTiO2-basedsystemsintheliterature[40–42].Itshould benotedthatthephotocatalyticactivityofAbTisystemsaretyp- icallylowerthanthatofaconventionalbenchmarkcatalystsuch asDegussaP25revealing%photonicefﬁcienciesof0.45and0.11 forNO₂(g)productionandNOxstorage;respectively.Thiscanbe attributedtothehigherSSAofP25(ca.50m²/g).

Afterhavingdemonstratedthephotocatalyticwaterpuriﬁca- tioncapabilitiesoftheAb-tempaltedTiO₂microspheresunderUVA irradiation,weperformedfurtherstudiesinordertoestablishthe photocatalyticactivityofthisnewfamilyofmaterialsinphotocat-

alyticairpurificationapplications.Alongtheselines,photocatalytic NO(g)oxidationandstorageexperimentswerecarriedoutusinga custom-madephotocatalyticflow reactorunderUVA irradiation [8–10].Fig.8presentsresultsofthesegasphasephotocatalytic activitytests.Theresultingtypicaltime-dependentconcentration profilesforthephotocatalyticNOoxidativestorageexperimentis alsoshownintheinsetofFig.8.InthehistogramsofFig.8,per centphotonicefficiencyvaluesfortotalNOxremoval(bluebars) andNO2production(redbars)areshown.Itisworthmentioning thatanidealphotocatalystforgasphaseDeNOxapplicationsshould exhibita highNOx(g)storage/removalefficiencyaswellaslow NO2(g)generation/releasecharacteristics.Photocatalyticoxidative

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Fig.8. PhotocatalyticNO(g)oxidationandstorageperformanceresultsobtainedviaUVAirradiationatroomtemperatureforbiotemplatedTiO2microspheresinitially calcinedatvarioustemperatures(insetshowsthetypicaltime-dependentconcentrationproﬁlesfortotalNOx(g),NO(g),andNO2(g)overAbTi-600.(Forinterpretationofthe referencestocolourintext,thereaderisreferredtothewebversionofthisarticle.)

storageofNO(g)includesoxidationsteps[11,34,43,44]involving theformationofNO2(g),wheretheeventualstorageofNOxspecies onthecatalystsurfacemayoccurintheformofchemisorbedNO, NO₂/NO₂⁻,N₂O,andNO₃⁻.Thus,maximizingtheoxidativeNOx storageatthesolidstate,whilesimultaneouslyminimizingthegas phasereleaseoftoxicNO₂(g)requiresoptimizationofthechemical, electronicandsurfacestructureofthephotocatalysts.

Alongtheselines,photocatalyticDeNOxperformanceofthesyn- thesizedAbTi photocatalysts were investigated asa function of thecalcinationtemperatureusedinthesyntheticprotocol,inan attempttomonitorthestructure-functionalityrelationships.Pho- tocatalyticactivitydatapresentedinFig.8canbeanalysedinthe lightofthesearguments(Figs.1–6).Itisapparentthatunlikethe liquidphaseRhB(aq)degradationresultsgiveninFig.7,suggesting AbTi-800astheoptimumcatalystintheliquidphase,Fig.8shows thatAbTi-800haslimitedphotocatalyticNOxabatementcapability ingasphase.Thisobservationmaysuggestrelativelydifferentreac- tionmechanismsandinvolvementofdissimilaractivesitesforthe photocatalyticliquidphasewaterpuriﬁcationprocessesascom- paredtothegasphasephotocatalyticDeNOxprocessesoccurring onthesamecatalystsurface.

The highest total photocatalytic gas phase activity can be assignedtotheAbTi-600catalystgivenin Fig.8 duetothefact thatthiscatalyst revealsmaximumNOxremoval efﬁciencyand maximum photocatalyticoxidation of NO(g) toNO₂(g). On the otherhand,AbTi-600shouldnotbeidentiﬁedasthephotocata- lystofchoiceduetoitshighNO2(g)releasetotheatmosphere.

Comparisonof theAbTi-600catalyst withAbTi-500 revealsthat theAbTi-500hasacomparableNOxstorageefﬁciencytothatof theAbTi-600catalyst,whileexhibitingmuchlowerNO₂(g)release.

Hence,AbTi-500canbeconsideredasthepreferablecatalyst in theseriesforgasphasephotocatalyticDeNOx applications.It is likelythatthegasphasephotocatalyticoxidationofNO(g)requires thepresenceoforderedanatasedomains,whilepreventionofthe NO₂(g)sliptotheatmosphererequiresaporous/highsurfacearea catalystthatcanoptimizecapture/adsorption/solidstatestorage ofthegeneratedNO2(g).Thisisconsistentwiththeobservation thattheAbTi-400catalystobtainedaftera low-temperaturecal- cinationstephaslimitedNOxremovalefﬁciencyaswellaslow NO2(g)production,duetothelackoforderedanatasedomainsand

presenceofsmallanataseparticlesanddisordered(amorphous) domains.Inotherwords,themainreasonforthepoorperformance ofAbTi-400seemstobeitslimitedphotocatalyticoxidationcapa- bilityratherthanitslackofsurfaceareaforNOxstorage.Incontrast, forthecatalystscalcinedatT≥700^◦C,themaincatalyticdisadvan- tagecouldbeshrinkingofthepollensathightemperatures(i.e.

decreaseintheavailablesurfacesitesforadsorptionandstorage ofoxidizedNOxspecies)anddecreaseinthenumberofexposed activesites,whichinturnhinderthestorageofphotocatalytically producedNO₂species,resultingindetrimentalNO₂releasetothe atmosphere.Comparisonoftheliquidphasephotocatalyticactiv- ityofAbTisystemswiththatofabenchmarkcatalyst(i.e.Degussa P25)revealsthatthelattersystemhasahigherphotocatalyticactiv- ity,where100%decolourizationefﬁciencycanbereachedafterca.

70min.Thisobservationcanbeassociatedwiththehighersurface areaofthelattersystem.

4. Conclusions

A novel biotemplate-based photocatalytic material platform wassynthesizedbyutilizingAmbrosiatriﬁda(Ab,Giantragweed) pollenastheinitialbiologicalsupportsurface.Structuralcharac- terizationofthesynthesizedbiotemplatedTiO₂microsphereswas performedusingSEM-EDX,Ramanspectroscopy,and XRDtech- niques.Photocatalyticfunctionality ofthesynthesizedmaterials wasdemonstrated both in gasphase (via photocatalytic oxida- tiveNOxstorage)as wellasin liquid phase (viaphotocatalytic Rhodamine B(aq)degradation)asa function ofthecalcination temperatureusedinthesyntheticprotocol.Optimumcatalystfor RhB(aq)photocatalyticdegradationintheliquidphasewasfound tobeAbTi-800,whiletheoptimumcatalystforgasphasephotocat- alyticoxidativeNOxstoragewasAbTi-500;emphasizingdifferent structural/functionalrequirementsfordifferentcatalyticreactions occurringonthesamecatalyticsurface.Thesyntheticapproach presentedhereinoffersnewopportunitiesforobtainingadvanced functionalmaterialswhichcanhavepotentialprospectiveapplica- tionsinmicroreactorsystems,adsorbents,drugdeliverysystems, catalyticprocesses,andsensortechnologies.

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Acknowledgments

EOacknowledgesﬁnancialsupportfrom“TheScienceAcademy”

(Turkey) through “Young Scientists Award Program (BAGEP)”.

AuthorsalsoacknowledgethescientiﬁccollaborationwithTARLA projectfoundedbytheMinistryofDevelopmentofTurkeyunder grantnoDPT2006K-120470.

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